This invention generally relates to the corrugated board industry and, more particularly, to an apparatus for manufacturing corrugated board that includes a preheater section and hotplate section. Specifically, the apparatus includes a variable heat transfer system for the preheater section and a passive hold-down mechanism for the hotplate section.
Corrugated board can be manufactured in many different widths and thicknesses. The thickness of the corrugated board is determined by the number of medians and liners in the board. First, corrugations or ridges are created in a median by passing the median through a corrugator. Then, an alternating series of liners and medians, with an adhesive between each layer, are brought together in a moving surface to form a corrugated board of desired thickness. The moving surface passes through an assembly line that includes the hotplate section, where heat and pressure are applied to dry the board and set the adhesive, and a cooling section, where the corrugated board is cooled. The moving surface is then cut and scored to make corrugated board of different shapes and sizes for boxes and other items.
Uneven moisture content in the source paper, which can cause portions of the board to shrink after the adhesive has set, is the principle cause of warping and the resulting waste encountered in manufacturing corrugated board. Accordingly, it is important to dry the source paper evenly before the medians and liners are brought together to form the board. To prevent warping, the source paper is passed through a preheater assembly that dries the source paper before it is processed by a machine for manufacturing corrugated board.
The preheater assembly typically includes a continuous web feeder, such as an unroller for feeding a continuous web of paper from a long sheet of paper wound around a core, a dryer, and a tension roller located between the feeder and the dryer for taking any slack out of the paper web. The dryer typically includes a large heated drum and two smaller idler rollers that keep the paper web in contact with the heated drum over a substantial portion of the circumference of the heated drum. From the dryer, the paper web travels to the machine for manufacturing corrugated board. Typically, the next stage in the assembly line is either a corrugator section, which flutes a paper web to create a median, or a singlefacer section, which places a layer of adhesive between a median and a liner.
In order for the preheater to dry the paper properly, the paper web must be held tightly against the heated drum. If slack develops in the paper web, air bubbles can form between the paper and the heated drum resulting in uneven moisture content in the paper exiting the dryer. Preventing slack from developing in the paper can be difficult, however, because the paper may have been rolled onto the core with uneven tension across the length of the paper roll. Typically, this causes one edge of the paper web to be taught while the other edge develops slack. This slack can remain in the web after it travels over the tension roller, over the first idler roller, and onto the heated drum, causing air bubbles or loose edges to develop between the web and the drum.
In one conventional moving-web slack-reducing system designed to overcome this problem, the paper web is fed over a vertically-actuated tension roller. The vertical actuators, one typically placed at each end of the tension roller, allow each end of the tension roller to be lifted and lowered a small amount in an attempt to take any slack out of the paper web. But this solution is somewhat wanting in performance because removing the slack from the web requires precise positioning of the actuators. In addition, the tension tends to vary quickly across the paper web as the paper is unrolled. Removing the slack from the web under these conditions therefore requires fairly rapid and precise adjustments of the actuators, which are difficult to achieve at a reasonable level of investment.
In order for the paper to be dried properly, the preheater must also remove wet streaks in the paper. Wet streaks in the paper result in uneven moisture content in the paper. Therefore, extra heat transfer is required between the areas on the paper with the wet streaks and the preheater. However, the tension roller, described above, only moves up and down and not in the crossmachine direction. No known mechanism exists for increasing the heat transfer between the paper and the preheater in the crossmachine direction to eliminate the wet streaks.
The hotplate section of the corrugated board manufacturing apparatus also includes the heated platform section, typically a series of steam chests, that heat the corrugated board to set the adhesive and to remove moisture from the medians and liners. An array of pressure applicators press the corrugated board against the heated platform to assist in moisture removal and heat transfer. The pressure applicators press the corrugated board against the steam chests to ensure adhesion across the entire width of the corrugated board to prevent blisters from forming in the corrugated board.
Because the steam chests tend to warp over time, usually with a sag in the middle, a rigid pressure applicator would crush the edges of the corrugated board and leave blisters in the middle of the board. Many machines are also configured to manufacture corrugated board of varying width. These machines should be capable of varying the pressure applied across the machine width because the edges of the corrugated board, which are only supported by adjacent corrugated board on one side, are easier to crush than the middle of the corrugated board. In addition, it may be desirable to vary the pressure in the cross-machine direction in response to variable moisture content in the board. Specifically, it may be advantageous to apply extra pressure to wetter areas of the board. Devices have been developed with complicated and expensive controls for applying variable pressure across the width of the steam chests (i.e., in the cross-machine direction).
In a typical configuration, the hotplate section of a machine for manufacturing corrugated board includes 16 steam chests that are 7.3 feet (2.2 m) wide and extend in combination about 21 feet (6.5 m) in the direction of machine flow. A row of eight pressure applicators may overlie each steam chest in the cross-machine direction. This allows pressure to be applied over more steam chests for thicker corrugated board and at higher machine speeds. For example, pressure may be applied over only four steam chests (i.e., one group) for single-median corrugated board, over eight steam chests (i.e., two groups) for double-median corrugated board, and over all sixteen steam chests (i.e., four groups) for triple-median corrugated board. In addition, to increase the production output of thinner gauges of board, the machine speed may be increased and pressure may be applied over more steam chests. The hotplate section thus includes a grid of pressure applicators including rows of applicators in the cross-machine direction and columns of applicators in the direction of machine flow.
The conventional configuration described above has certain shortcomings when used to manufacture thick corrugated board, such as triple-median board. Namely, it is difficult to transfer heat from the steam chests all the way through to the top layers of the board. The thicker corrugated board therefore requires more time in the hotplate section to bring the temperature of the top layers of adhesive to the required setting temperature. It is also difficult to remove moisture from wet areas in the top layers, which can cause the board to warp as it dries. To counteract these problems, the speed of the board must be slowed considerably to ensure adequate moisture removal from the top layers of the board and adequate heating of the top layers of adhesive. This decrease in the speed of the assembly line decreases the production output and increases the cost of the thick corrugated board. Another shortcoming is that it is expensive to have a control system to vary the pressure applied to the corrugated board that requires continuous monitoring by an operator. This continuous monitoring in the hotplate section while the machine is manufacturing corrugated board also increases the cost of the corrugated board.
There is a need for a more effective system and method for removing moisture while in the preheater section, before the sheets have been brought together into a formed board. Also, the there is a further need for a more efficient system and method for compensating for variations in the contour of the surface of the steam chests in the hotplate section so that the fabricated corrugated board is cured evenly.
The present invention alleviates or solves the above-described problems in the prior art by providing an improved corrugated board manufacturing apparatus. This apparatus provides a preheater including a variable heat transfer system for removing moisture from the paper sheeting by applying variable pressure in the cross-machine direction. Also, the present apparatus provides a passive, segmented hold-down mechanism that compensates for variations in the contour of the surface of the steam chests. The hold-down mechanism facilitates curing of the adhesive in the formed board. A particular cross-machine pressure profile is desirable across the surface of the steam chests. The variable heat transfer system and the hold-down mechanism each decrease the amount of wasted corrugated board, damaged as a result of uneven moisture content, resulting in substantial cost savings. The use of the hold-down mechanism also results in substantial cost savings because there are no active controls in the hotplate section and because it is cheaper to make the machine with such a hold-down mechanism.
In accordance with one aspect of the present invention, a preheater assembly having a variable heat transfer system includes a plurality of actuators for urging a continuous web of material toward a preheater drum. The actuators press against a sacrificial material which, in turn, presses against the web on the preheater drum. The actuators and sacrificial material are arranged so that variable pressure is placed upon the web in the cross-machine direction through actuation of the actuators. By applying pressure against the web on the preheater drum, air gaps between the web and the preheater drum and the wet streaks in the web can be removed. The heat transfer between the web and the preheater drum is increased by applying pressure against the web on the drum.
More particularly, the actuators may be controlled by a controller. Moisture content sensors are arranged to measure the moisture content of the paper sheeting immediately before the preheater drum. The actuators are responsive to the signals received by the controller such that the paper sheeting is urged against the preheater drum in areas of high moisture content. Additional sensors may also be used immediately after the preheater drum in combination with the other sensors placed ahead of the preheater drum. Alternatively, sensors may be utilized only immediately after the preheater drum. In such case, the controller is referred to as a feedback controller.
In accordance with another aspect of the present invention, a passive, segmented hold-down mechanism for the hotplate section includes a plurality of independently weighted feet for forming a variable pressure profile in the cross-machine direction upon the web of corrugated board. The independently weighted feet compensate for variations in the contour of the surface of the hotplate section. Each of the feet may include a mechanical stop for stopping the foot above the underlying hotplate section. Actuators may be used to raise or lower the feet between no-load and full-load positions. However, once the hold-down mechanism is set for the desired pressure profile, the hold-down mechanism is otherwise free of controls.
The foregoing has broadly outlined some of the more significant objects and features of the present invention. These should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or by modifying the disclosed embodiments. Accordingly, other objects and a more comprehensive understanding of the invention may be obtained by referring to the detailed description of the preferred embodiment taken in conjunction with the accompanying drawings, in addition to the scope of the invention defined by the claims.